Used appropriately, a fume hood can be a very effective device for containment of hazardous materials, as well as providing some protection from splashes and minor over pressurizations. Even when used as designed, the average fume hood does have several limitations.

• Projectiles: A fume hood is not designed to contain high velocity releases of particulate contaminants unless the sash is fully closed.
• Pressurized systems: Gases or vapors escaping from pressurized systems may move at sufficient velocity to overcome the capture velocity and escape from the fume hood.
• Explosions: The hood is not capable of containing explosions, even when the sash is fully closed. If an explosion hazard exists, the user should provide anchored barriers, shields or enclosures of sufficient strength to deflect or contain it. Such barriers can significantly affect the airflow in the hood.
• Perchloric Acid: A conventional fume hood must not be used for perchloric acid. Perchloric acid vapors can settle on ductwork, resulting in the deposition of perchlorate crystals. Shock-sensitive perchlorates can accumulate on surfaces and have been known to detonate on contact, causing serious injury to researchers and maintenance personnel. Specialized perchloric acid hoods, made of stainless steel and equipped with a wash-down system must be used for such work.

• Connections to the Exhaust System: Occasionally, a researcher may need local exhaust ventilation other than that provided by an existing fume hood. A new device may not be connected to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device.
• Microorganisms: Work involving harmful microorganisms should be done in a biosafety cabinet, rather than a chemical fume hood. See the Biosafety Manual for more information.
• Highly Hazardous Substances: A well designed fume hood will contain 99.999 - 99.9999% of the contaminants released within it when used properly. When working with highly dangerous substances needing more containment than a fume hood offers, consider using a glove box.
• Pollution Control: An unfiltered fume hood is not a pollution control device. All contaminants that are removed by the ventilating system are released directly into the atmosphere. Apparatus used in hoods should be fitted with condensers, traps or scrubbers to contain and collect waste solvents or toxic vapors or dusts as appropriate.

When the exhaust is running, it sucks in volumes of air through the open side (never run the exhaust with the hood completely closed) and vents it through the duct. The duct may throw the fumes right outside the room in the air, or may run it through a filtration system, depending upon the severity of the experiment.

Some advanced laboratory fume hoods have microcontrollers inside them, which you can use to control the exhaust. You may set various suction levels for given setups and later use these quick setups for particular experiments. You can also control the fan/pump power during the experiment too, and the lights as well.
 
Inside the fume hood, there are facilities to let you work. For example, there is almost always some good lighting system in the cabinet. There should be LPG and water connections in the hood, and a drain too in case of spills. 
The air suction system is generally placed on top of the box or on the backside (the side towards the wall). This system is generally made up of a strong exhaust fan or air pump and accompanying air duct. The strength of the pump lies in the airflow strength starting from 4 to 8 meters per second to more.